Their quick efforts yielded unprecedented data when the mountain finally blew its top in March 2009.

McNutt, of the Alaska Volcano Observatory, had observed similar sparks during a 2006 eruption of Alaska's Augustine Volcano. The Redoubt Volcano data confirms the lightning's existence, he said.

Lightning, by Any Other Name

The newfound bolts join two other types of volcanic lightning, McNutt said: Large, spectacular "natural fireworks" that sometimes accompany eruptions and an intermediate type, which shoots up from a volcano's vents and reaches a length of about 1.8 miles (3 kilometers).

Both types of bigger, more obvious bolts occur when water droplets and ice particles interact with the volcano's plume of electrically charged ash, creating a sort of "dirty thunderstorm," McNutt said (see pictures of Redoubt Volcano's large lightning storms).

It's unknown how the smaller sparks form, though one possibility is that electrically charged silica—an ingredient of magma—interacts with the atmosphere when it bursts out of Earth's crust, he said.

Still, it's hard to say if the sparks indeed represent a new type of lightning, noted Martin Uman, a lightning expert at the University of Florida in Gainesville. That's because lightning—basically any discharge of electricity—has no scientific definition.

That ambiguity might be a good thing, he added: "It's more sexy to call it lightning, and scientists feel that way too."

Lightning Safety Boon

No matter what you call them, the tiny sparks near volcanoes' vents may offer a safety benefit, added Uman, who was not involved in the Redoubt Volcano study.

When a volcano gives off a hint of an impending eruption—called a precursor event—scientists could set up instruments near the vents to detect sparks as an eruption begins, which would then alert officials even sooner, he said.

Such a warning could be critical for air traffic, since ash emitted by volcanoes is especially hazardous to jet engines.

There's also an aesthetic pleasure in watching lightning events: Any kind of volcanic lightning is just "supergorgeous," Uman said. "It's one of our best natural phenomena."